This invention relates to a device for reducing the leakage noise of a power converter, which, in particular, performs an AC-DC-AC type of double conversion, which is compact, and effectively reduces the noise leakage produced by an inverter of an air conditioner, for example.
Recently, air conditioners in which the speed of the compressor motor is varied by an inverter, which is one type of power converter, and which are energy efficient and very practical compared with the prior art air conditioners in which the compressor motor is controlled by turning it on and off, have very rapidly become widespread.
FIG. 1 shows this kind of air conditioner system in which the outdoor unit contains an inverter section and a compressor, and the speed of the compressor motor is controlled by a 30-90 Hz dummy sinusoidal alternating current produced in the inverter section. In the inverter section in this kind of a structure, the alternating current (power source frequency) is converted to direct current and back to alternating current (variable frequency) by using semiconductor rectifiers and switching elements.
FIG. 2 is a basic circuit diagram of the inverter section of the air conditioner. AC power source 1 is connected in parallel to an inverter rectifier circuit 3 via the inductive load of a fan motor 9, and a four-way valve coil 10, etc. and a reactor 2 for improving the power factor. The inverter rectifier circuit 3, which is a voltage doubler rectifier circuit comprising two diodes and two capacitors, converts a 100 V AC voltage from the AC power source 1 to about a 250 V DC voltage. This DC voltage is smoothed by capacitor 4 and supplied to inverter circuit 5. In this case, the inverter circuit 5 is a three-phase circuit having six switching transistors. The operation of each of the six transistors is controlled by a control circuit (not shown) and the DC voltage is converted to a dummy sinusoidal AC voltage.
In this electrical circuit, which forms the inverter section, the double conversion of AC-DC-AC (variable frequency) is carried out until a three-phase AC current is supplied to the compressor motor 6 from the AC power source 1. At the time of these conversions, a noise voltage, which results in radio noise, is generated and leaks to the AC power source side. Consequently, as can be seen in the prior art circuits of FIGS. 3 and 4, a noise filter 7 for reducing the leakage noise has been included. In the circuit of FIG. 3, noise filter 7 has been inserted between the inverter circuit 5 and the rectifier circuit 3. This noise filter 7 is constructed of two capacitors 20 and two inductors 21, which are coupled by a ferrite core 21a. Because the current value between rectifier circuit 3 and inverter circuit 5 is about half the value on the AC power source side, the diameter of the wires used in the inductors 21 inside the noise filter 7 may be small and the inductors 21 and ferrite core 21a can be made compact. Therefore, because the major portion of the noise that is generated is based on the switching operation of the transistors inside the inverter circuit 5, which located after noise filter 7, it is possible to reduce the noise to a certain extent.
Noise, however, is not generated only by the inverter circuit 5 but also by the rectifier circuit 3 in the stage before noise filter 7.
This noise from the rectifier circuit 3 is based on the commutation operation and the amount generated, when compared to that generated from inverter circuit 5, is relatively low, but is still enough to inflict actual damage to a radio receiver which is plugged into the same AC power source. Using this kind of circuit discribed above, the amount of noise reduction is insufficient.
The electrical circuit shown in FIG. 4 has a noise filter 8, comprising four capacitors 22 and two inductors 23, which are coupled by a ferrite core 23a, inserted between the power-factor improving reactor 2 and rectifier circuit 3.
In this case, there is a redction in the noise from both the inverter circuit 5 and rectifier circuit 3. However, the noise filter 8 is inserted on the 100 V AC power supply side so the current flowing in the inductors 23 is large. Thus, the wires used in the inductors 23 must be large as must be the ferrite core 23a. The noise filter required to reduce the noise level to 55 dB is exceedingly large. Furthermore, it is necessary to increase the capacity of the ground capacitors 22 in the noise filter 8. The noise current leaking to ground from this capacitors is very large, making the device impractical.